The subject matter disclosed herein relates generally to detecting fault conditions in a motor drive and, more specifically, to detecting a ground fault in a motor drive system.
Common AC motors use three-phase electrical power connected to the stator windings of a motor to run the motor. Each stator winding is connected to a different conductor from a three-phase power source, in which each conductor delivers a different phase of the electrical power to the motor. The three-phase power source may be a direct connection to line power, but more commonly, the motor is connected to an adjustable speed motor drive (ASD). The ASD allows for speed control of the motor not available by connecting the motor directly to line power.
As is known in the art, there are many electrical topologies for ASDs used to convert the fixed voltage and frequency from the line input into a controlled voltage and frequency output for a three-phase motor. One common topology includes a rectifier section which converts the line power into a DC voltage used to charge a DC bus section of the ASD. An inverter section then uses a switching algorithm, typically pulse width modulation (PWM), to convert the DC voltage from the DC bus into a variable voltage and frequency output to the motor. Controlling the variable voltage and frequency output to the motor controls the speed at which the motor rotates.
In another topology, a single rectifier section may be configured to supply a DC voltage to multiple inverter sections. The inverter sections may be distributed on or near the motor the inverter section is intended to control. The rectifier section again converts the line power into a DC voltage for a DC bus, but each inverter section is connected to and receives power from the same DC bus. The components of the rectifier section are, therefore, selected such that their power rating is sufficient to supply power to all of the inverter sections connected to the DC bus.
Each of the motors connected to one of the inverter sections typically includes a ground connection on the chassis of the motor. A conductor is run between the ground connection on the motor and a ground connection on the inverter section. The ground connection on the inverter section is, in turn, run to a centrally ground connection such that the chassis of the motors are held at zero voltage potential to minimize the risk of electrical shock or equipment damage. Common wiring practice utilizes a four-wire cable or bundle of conductors that includes three hot conductors and one ground conductor which runs between the inverter section and the motor. Each phase of the output voltage supplied by the inverter section is connected to one of the hot conductors and the ground conductor is connected between the ground connection on the motor and the ground connection on the inverter.
Although each conductor is typically labeled, the potential exists for one of the hot conductors to be crossed with the ground conductor between the inverter section and the motor. Such a wiring error causes a ground fault at the output of the inverter section and will likely result in a failure in the motor, the inverter section, or in the rectifier section supplying power to the inverter section should the input power and, consequently, normal DC bus voltage be applied to the motor drive system with the incorrect wiring. Further, a failure in the system, such as a short circuit in the motor windings or between conductors connecting the motor to the inverter, can also cause a ground fault.
Thus, it would be desirable to provide a system to detect a ground fault prior to applying the input power to the motor drive system.